Advances in synthetic methodologies have enabled the evolution of structurally complex polymers with control over size (molecular weight) and architecture (e.g. statistical, alternating, block), affording the achievement of advanced structures within very narrow limits of molecular weight and end-group fidelity. Attention has turned toward functional polymers, with an ever-increasing range of capabilities that push the limits of current approaches. Transition metal-mediated polymerizations have played a critical role in this context; however, there is an ongoing need for new reactions that yield advanced macromolecular structures with enhanced properties and performance. The ultimate viability of these approaches will also rely on atom economical transformations, insensitivity to O2 and H2O, high functionality tolerance, increased molecular complexity, and orthogonality to other transition metal catalyzed methodologies and polymerization protocols.
Homogenous gold (Au) complexes have emerged as excellent catalysts in numerous transformations involving the activation of unsaturated carbon-carbon (C—C) bonds towards the attack of a wide variety of nucleophiles. Relativistic effects in Au provide superior Lewis acidic behavior and preferential reactivity with “soft” species such as π-systems. Au complexes have a particularly strong affinity for alkynes and allenes, even in the presence of other functional groups, affording very selective transformations and access to highly complex molecules. These complexes can also stabilize cationic reaction intermediates formed by backdonation. The non-classical nature of these intermediates, together with a low propensity toward β-hydride elimination, frequently results in excellent reactivities and selectivities. These features coalesce to afford transformations which are not possible with other transition metals and provide a rapid increase in structural complexity starting from very simple substrates. Other advantages include reduced oxophilicity, tolerance toward O2 and H2O, and the use of very mild reaction conditions.
Cationic Au species are regarded as the most powerful catalysts for the electrophilic activation of alkynes/allenes/alkenes towards a wide variety of nucleophiles. Although there are many types of transformations catalyzed by homogeneous Au complexes, a majority of them proceed through very similar mechanistic steps. Firstly, a cationic Au complex acts as Lewis acid to activate C—C multiple bonds and form a η2-complex. Simultaneously, Au can also complex with other components (solvent, nucleophiles, products, additives, etc.) in the reaction system. Slippage of Au will form a very reactive intermediate, which is eventually attacked by the nucleophile. Finally, protodeauration (addition of a proton to the site where the removal of Au takes place) regenerates cationic Au to give the final product.
Homogenous Au catalysis has received significant attention in the organic literature leading to the discovery of a remarkable amount of new, synthetically useful transformations and access to unprecedented molecular architectures. While immense progress has been made in the field, there are very few examples of homogenous Au catalysis and its relation to polymer chemistry. The full potential of gold catalysis to construct novel macromolecular architectures has not been realized and is only in its infancy. As a relevant example, homogenous Au catalysis was used to create an original macromolecular skeleton using the first ever carbene-to-olefin polymerization. Monomers incorporating both a propargylic ester and an alkene moiety were polymerized yielding novel polymers possessing an original cyclopropyl-vinylester-phenyl skeleton. The polycyclopropanation process demonstrates the only example to date of an Au-catalyzed redox neutral mechanism to access a unique macromolecular architecture.
The Au-catalyzed intermolecular hydroarylation of alkynes leads to 1,1-disubstituted alkenes. An extension of the above methodology involves the sequential reaction (“polyhydroarylation”) of bifunctional or multifunctional monomers.